The present invention relates to switching regulators and, more particularly, to a switching regulator in which regulation or adjustment of output voltage is possible over a relatively wide range.
A conventional switching regulator has been arranged, for example, as shown in FIG. 2 of the attached drawings. Specifically, the switching regulator 1 is arranged such that D.C. voltage inputted from a D.C. power source (not shown) through a pair of input terminals 2 and 2 is applied to a primary coil 4a of a transformer 4 through a switching transistor 3, and a pair of output terminals 6 and 6 are connected to a secondary coil 4b of the transformer 4 through a rectifier smoothing circuit 5. When output voltage occurring between the output terminals 6 and 6 is inputted to an output-voltage control circuit 7, the output-voltage control circuit 7 controls oscillation of the switching transistor 3 due to a drive control circuit 9, through a photocoupler 8, on the basis of the above-described output voltage.
In the output-voltage control circuit 7, the aforesaid output voltage is directly inputted to one of a pair of input terminals of a comparator 7a, while the aforementioned output voltage is inputted to the other input terminal of the comparator 7a through a power source 7b variable in voltage. That is, voltage occurring at both ends of the power source 7b is inputted to the other input terminal of the comparator 7a, as a predetermined reference voltage. Accordingly, the comparator 7a compares the aforementioned output voltage and the reference voltage with each other to output a signal. The output signal from the comparator 7a is inputted to one of a pair of input terminals of the other comparator 7c, and a pulse signal from an oscillation circuit 7d is inputted to the other input terminal of the comparator 7c. In the case where the output voltage is higher than the reference voltage, a signal of an H-level is outputted from the comparator 7c, while, in the case where the output voltage is lower than the reference voltage, a signal of a L-level is outputted from the comparator 7c.
With the arrangement described above, in the case where the output signal from the comparator 7c is the H-level, a luminous section of the photocoupler 8 does not emit light or luminesce, because voltage required for luminescence is not applied to the luminous section. Accordingly, a signal is not inputted to the drive control circuit 9. On the other hand, in the case where the output signal from the comparator 7c is the L-level, the luminous section of the photocoupler 8 luminesces, because voltage required for luminescence is applied to the luminous section. Accordingly, a signal is inputted to the drive control circuit 9. Thus, the drive control circuit 9 controls oscillation of the switching transistor 3 on the basis of the above-described pulse signal. In this manner, the output voltage can be maintained to predetermined voltage.
However, the switching regulator 1 described above is arranged such that the output voltage is substantially equalized to predetermined voltage which is set beforehand. The switching regulator 1 is provided with the power source 7b variable in voltage for regulating the reference voltage. In the case, however, where the output voltage shifts from the predetermined voltage, the power source 7b variable in voltage is one for regulating the output voltage to the predetermined voltage, and has a regulating range or region which is relatively small or narrow. Accordingly, it is difficult to regulate the output voltage over a relatively wide range. For this reason, there is the following problem. That is, in the case where the output voltage is modified or altered to output voltage which exceeds or is out of the regulating range, the switching regulator per se must be replaced by another one.